This invention relates to operational amplifiers for use in electronic apparatus wherein the amplifier is required to respond to rapid large shifts in the input signal. Such applications are commonly found in the drive circuits for cathode ray tubes, particularly in those instruments wherein the cathode ray tube displays data which is collected at one rate and is speeded up and compressed for display purposes. Such instruments are common in cardiac monitoring.
As may well be expected in monitoring of physiological signals, it is a requirement that the output voltage or current of the operational amplifiers making up the current drives for the display tube bear as precise a relationship as possible with the input voltage or current (e.g. the physiological information). It is well-known that amplifiers in general have limits on how fast their output may respond to an input command. This limit is generally spoken of in terms of slew rate and is defined usually as the maximum rate of change of the output voltage of an amplifier in response to an input offset voltage.
The limitations on slew rate are characteristically due to the limited amount of power available within an operational amplifier system to change the energy stored in the load being powered by the amplifier. As may well be expected, the problems of maximizing slew rate become particularly critical when the load is largely reactive, i.e. inductive or capacitive. In those installations when the operation amplifier is driving the yoke of a cathode ray tube, this critical problem is observed since the amplifier load is largely inductive. The problem is further complicated when, as in the case of cardiac monitors, information is gathered at one rate and displayed at a rate of, for example, 1000X the collection rate. As may be appreciated, in such instances when there is a large swing from one voltage level to a second level, the recovery problems for the amplifier following the rapid slew of voltage are substantial.
In such instruments as multi-channel, non-fade displays which are used as bedside monitors in hospital intensive or coronary care units, physiological conditions normally expressed in milliseconds become, by virtue of data compression for presentation, in the microsecond range. Others have solved the demanding time requirements of these physiological monitors by using a TV-type raster scanning and providing the display through the modulation of the beam intensity, as opposed to using a true X-Y deflection scheme. In such TV-type monitor installations, a comparatively small amount of energy change is necessary to brighten up the spot of the beam being scanned in one of the traditional raster patterns so the slew problem is not exhibited. Likewise, there is ample prior art in the building of raster generators to power such systems.
There are certain advantages, however, to a true X-Y presentation of the physiological signals on a CRT monitor. Among these are included the continuous display of the information by a beam which is writing a continuous trace on the tube face. It is held in some medical circles that such a display is more stable and easy to interpret as by giving a more crisp definite display on the cathode ray tube.
It must be appreciated, however, that in order to insure a clear, crisp accurate presentation or display of the physiological signals upon the screen, that the slew rate of the Y-axis deflection amplifier must be equivalent to the system demand. In such displays, the input signal level is traditionally in the region of one millivolt and the response time, as previously mentioned, is a compressed data display in the order of microseconds. This low level input signal must be converted into a yoke drive current in the order of one amp. Thus, it may be seen that if overshoot, which is a common observable characteristic of an amplifier operated beyond the slew rate capability, is to be kept to an acceptable minimum, for example, 50 microvolts, the problem faced is the design of such an amplifier wherein a response time of approximately 50 nanoseconds is achieved.
I have determined that an operational amplifier for driving a highly reactive load such as the Y-axis of a CRT can be modified with additional circuit elements so as to anticipate potential overshoot when operated in slew. By anticipating the otherwise normal overshoot, the time required for the amplifier to stablize after a slew condition may be reduced and additionally the overshoot itself may be minimized or eliminated.